The goal of this work is to understand the replication cycle of murine leukemia viruses (MuLV's), and the mechanism of Fv-1 restriction, a naturally occuring cellular system with specifically inhibits the replication of certain MuLV's. The studies use a number of cell culture techniques for quantitation of virus infectivity. Defective variants of MuLV have been isolated and characterized. Biological techniques for the study of these variants have been developed, including methods for detecting and rescuing defective MuLV and transmitting it to new host cells. These variants will be used for genetic studies on MuLV replication.